Telephone ringing system



July 27, 1937. 1.. J. STACY 2 88, I TELEPHONE RINGING SYSTEM I Filed April 17, 1936 2 Sheets-Sheet 1- FIG , ATTORNEY July 27, 1937. 1.. J. STACY TELEPHONE RINGING SYSTEM Filed April 17; 1936 2 Sheets-Sheet 2 FIG. 2

F/GLB I GAS F/L LED,

4/M ans FILLEDy ATTORNEY Patented July 27, 1937 UNITED STATES PATENT OFFICE.

TELEPHONE RINGING SYSTEM Leland J. Stacy, Caldwell, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application April 17, 1936, Serial No. 74,836

4 Claims. .(Cl. 179-86) nating current wave with its neutral axis displaced from the central line of the wave by the amount of the battery voltage. Such an arrangement is disclosed in the patent to Drake 763,970 of July 5, 1904.

At each station a relay is bridged across the line in series with a condenser and a ringer is connected from one side of the line or the other to ground over a normally opened contact of its associated relay as shown in the Thompson et a1. Patent No. 644,647 of March 6, 1900. When the superimposed current source is connected to the line, all the bridged relays operate but current only flows through the two ringers which are connected to the live or ungrounded side of the line as the other side of the line is grounded at the central office thereby shunting the ringers be biased, i. e. the armature must be held by a spring to one side or the other depending on the direction of the current pulsations to which it is desired that it shall respond.

Systems of this character are so well known that a more complete description is deemed unnecessary.

A more recent suggestion relating to the connection of such polarized and biased ringers to a line to selectively respond to superimposed pulsating currents, as before described, is contained in the patent to Norton and May 1,517,857, dated December 2, 1924 which proposed to replace the bridged relay and condenser combination, shown in the Thompson et a1. patent with a gas discharge device containing two relatively closely spaced electrodes which device is serially connected between the respective ringer and the line, such devices having the characteristic of preventing the start of any current flow in the ringer at voltages below the peak ringing voltage, i. e. of the order of 117.to' 126 volts, but

when the ringing source is connected, those de-- vices connected to the line side of the line allow the ringing current to flow through the associated ringers.

As such two-element devices break down on,

and conduct, current flowing in either direction, substantially equally, therefore, it is necessary to retain the polarized and biased types of ringers in order to insure that only one ringer responds to a particular direction of the ringing current.

In the patent to Norton, 1,778,768 issued October 21, 1930 this problem was attacked by including in the discharge tube ringer circuit of Patent 1,517,857, a rectifying device.

A feature of the present invention resides in the substitution, for the two-element device of Norton et al., of a new type of gas discharge device employing three electrodes one of which is designated as the control electrode, which device is so constructed and connected that an applied ringing voltage of the proper value first causes a discharge to take place between the control electrode and the cathode thereby causing ionization of the gas within the device whereupon current therein, and only then, flows freely in the direction of the anode to the cathode and not at all, or at greatly reduced volume, from the cathode to the anode.

By employing such a discharge device and a high impedance in series with the connection I to the control electrode to limit the breakdown discharge. 1 r

In certain instances where selective party line ringing, of the foregoing character, is employed it is desired to install coin boxes at the subscribers station which are controlled, for the purpose of collecting coins, by negative'polarity direct current of the order of volts transmitted over the tip conductor of the line to ground through the coin box magnet. In this case if discharge devices, of the character described above, are substituted for the present relayand condenser arrangement, the two devices poled with their anodes to ground and cathodes to the line, which may be referred to as the negative devices, may break down when the negative 110-volts coin collect current is connected to the-tip of the line. The negative device connectedto the tip breaks down directly therefrom and the negative device connected to the ring conductor breaks down due to the coin voltage on the tip which passes through the subscribers telephone set to the ring conductor.

To overcome this situation it is proposed to connect the two negative station discharge devices, not to ground as in non-coin box lines but, to the opposite side of the line from which the cathode of the device is connected. In this case the device gets its ground over the side of the. line which is grounded at the central ofl'ice when the ringing current is connected and only then. A subordinate feature therefore resides in substituting, at the negative stations, a connection from the control electrode to the opposite side of the line from the side the cathode is connected, in place of local ground at the station. In systems, where ringing current is intermittently and automatically connected to the"'subscriberfs line .at regular intervals until the subscriberanswers (known as machine ringing), a source of direct tripping current is connected across. the line :during the silent interval, 1'... e. the interval-between rings, .for the purpose of operating a tripping relay at the central omce to disconnect the ringing current when the called subscriber answers. The voltage of, this source of direct current may vary in different exchanges and incertain instances it may be as high or higher thanjthe breakdown voltage of the discharge devices whichit is desired to employ.

In this case it may ;be desirableto prevent breakdown of, the discharge devices during the silentinterval which is accomplished by shuntarranged for full selective ringing in which a three-element gas-filled tube of the cold cathode type, having a rectifying characteristic, is employed in series with the associated ringer to ground;

Fig. 3 is identical with Fig. 2 except that in the case of the negative stations the control cathode of the gas-filled tube is connected in each case to the side of the line opposite to that over which ringing current is transmitted, in-

stead of to local ground; and W Fig. 4 is identical with Fig. 3 except that a high impedance is connected betweenthe two cathodes of each of the four tubes of the four party'full selective line.

Referring to Fig. 1 of the drawings, which shows only such essential details as are necessary for an understanding of the method of applying selective ringing current to line s arranged in accordancewith the present invention, 23 is a source of alternating ringing current of any desired frequency, for example, 20 cycles, which is,

selectively supplied to contacts of the ringing relay I! through either'the polarizing battery PB- or PB+, 'correspondingbrushes of the in-;

terrupter I, windings oi relay [8 and contacts of relays 8, 25 and 21 as determined by the setting of; the individual party ringingkeys KW,

and KM. Fig; 2 represents, as before mentioned, a subscribers lineequipped withiour four-party: selective ringing subscribers sets arranged .to

operate on polarized ringing current, which sets have been modified from the standard arrangement to the extent that three-element gas-filled tubes of the cold cathode type ZQW, 29R, ZBJ and MM have been connected, in accordance with this invention, in such a manner that the respective ringers of the W, R, J and M sets respond only in accordance with the setting of the ringing 'at' the 'central ofiice. Fig. 3 also represents afour-party selective line with similar gas-filled tubes 30W, 3BR, SEi-J and 30M connected so that the respective ringers function as in Fig. 2, but alsoconnected, in accordance witha subordinate feature of the invention hereinbefore mentioned, so that, when negative coin current is connected to the tip of the line, the tubes 351W and 36J "at negative stations W and J do not break down... Fig.4 also represents a four-party and $8M connected, in accordance with another subordinatefeature of the invention, before mentioned, with a high impedance MW, llJ, MR and MM shunting the cathode elements of the respectivetubes, said impedance being provided to prevent the breakdownof the tubes when machine ringingris usedrand'the tripping battery voltage of the so-called silent interval exceeds the normalbreakdown voltage of the tube.

The circuitdetails and method of operation of these selective ringing sets, except as changed by the gas-filled tubes, are well known and described in the patent to Thompson et 9.1. 644,647, patented March 6, 1900.

The invention will probably be best understood from a description of the operation of the circuit shown in the drawings and it will therefore be first assumed that the operator having control of the automatic or machine ringing cord circuit of Fig. 1 desires to ring the bell of the subscriber W on line Ll of Fig. 2. To accomplish this the operator will insert the plugl in the jack 2 of the subscribers line and press the key KW associated with the cord circuit.

Insertion of the'plug l in the jack 2 completes an obvious circuit for operating the sleeve relay 3, which in operating completes circuits for operating start relay 4 and relay;5 which can be traced from battery, contacts l9 and 6 of relay 3,

. winding of relay 4 and contacts I of cut-off relay 8 to ground, and from battery, contacts 5% of relay 3, contacts ID of relay 8 and Winding of relay 5 to ground.

When key KW is actuated an obvious circuit is closed for operatingrelay H which thereupon j locks up to ground under control'of relay 5 in thefollowing circuit, battery, winding and contacts .|2 of relay-ll and contacts I3 of relay 5 to ground. Relay II also completes, at its contacts M, 'a circuit for operating relay l5 which can be tracedfrom battery, winding of relay [5, contacts I4 and I2 of relay H and contacts l3 of relay 5 to ground.

Relay I5 in operating completes a circuit at its left-hand contacts, ,for operating relay [5 which canbe traced from battery, winding of relay l5, left-hand contacts of relay I5 and contacts l3 of relay 5 to ground.

The operation of relay I5 also completes a circuit .to operate ringing relay I! which can be traced from battery, right-hand contacts of relay:, l5, left-hand contacts of relay 5, left-hand contacts of relay' l, winding of relay l1 and righthand normal contacts of tripping relay I8 to ound-.- Y

When relay l1 operates it connects the ringing leads 43 and 44 to the tip and ring of the cord and locks itself operated, at its contacts 2|, under control of relay 3.

The operation of relay l6 completes a circuit for operating relay 8 which can be traced from battery, contacts 69 of relay 3, winding of relay 8, contacts 20 of relay 4 and contacts of relay It to ground. Relay 8 in operating locks up at its left-hand contacts to ground under control of the sleeve relay 3, by opening its contacts, 1, it releases relay i, and by opening its contacts l releases relay 5.

Relay in releasing opens the holding path for relays ll, 55 and i5 which relays in turn release.

It will be noted that relays 8 and I! are now held operated under control of sleeve relay 3.

When relay 8 operated it also closed its righthand contacts 22 thereby connecting negative pulsating current, generated by superimposed a1- ternating ringing current supplied by the grounded source 23 on the battery PB, to the ring conductors of the cord and line in a circuit including contacts of the ringing interrupter I, left-hand winding of tripping relay l8, contacts 22 of relay 8, contacts 24 of relay 25, contacts 26 I conductor of the line Ll to the W station and over conductor ZlW to the cathode element 22W of the gas-filled tube 29W. A potential difference is accordingly established between the cathode element 22W and the control cathode element 23W which is connected through high control resistance 24W to ground. In similar manner the potential on the said ring conductor of line Ll also establishes a potential between the cathode elements of the tube 28R, at the positive station also on the ring conductor.

Battery PB is connected in the circuit with its n-egati"e pole toward the line and its positive pole toward the grounded alternating current source 23 and consequently the negative halves of the alternating current waves will be increased and the positive half waves decreased, thus supplying what is termed a negative pulsating current to the ring conductors of the line. This method of generating a so-called pulsating current for selective ringing purposes by superposing alternating current on a direct current source was described in Patent 763,970 to T. C. Drake dated July 5, 1904.

This so-called pulsating current is in fact alternating current with its neutral axis displaced from the center line of the wave by the amount of the battery voltage. On the basis of the before given description, assuming that ringing current has been connected to the ring conductor of line Ll, when a certain point on the wave of the said pulsating current potential is reached, ionization of tubes 253W and 28B is effected. This point of ionization is controlled by the impedances MW and 26R connected to cathode elements 23W and 23R of tubes 29W and R, re-

spectively. In practice it has been found convenient to have the tubes designed to break down on an ionization potential of approximately 70 volts, although any other feasible voltage that is suitable for the conditions encountered may be used. As soon as ionization takes place there is a flow of current at station W from positive ground through the winding of the ringer W to the anode element 26W of tube 28W, thence through tube 28W to cathode element 22W, conductor MW and back over the ring conductor path before traced. Prior to ionization the breakdown voltage between anode and cathode elements may be of the order of 150 volts; after ionization the voltage between cathode and anode may be reduced to approximately 80 volts before deionization takes place. It is to be understood, however, that any other feasible voltages required by a particular party line installation may be used.

The surface area of the anode element 25W of the tube 20W-is small. compared with that of the cathode element 22W. This anode element may be essentially a wire of small diameter. When the cathode element 22W isaccordingly subjected to a breakdown negative potential, an appreciable'quantity of electrons is emitted from" its comparatively large surface and there is a corresponding flow of current through the tube from the anode element to the cathode element. This flow of current, due to its pulsating character, causes the ringer 25W to respond in unison with the negative waves of ringing current transmitted over the ring conductor of line Ll When the ringing current wave passes through zero, from negative to positive, tube 20W is deionized and the voltage of the positive half-wave, reduced as it is by the voltage of the opposing battery PB, is insuiiicient to cause reionization and the current in the anode-cathode circuit remains at zero. Consequently, the ringer 25W responds only to current during the negative half of the ringing current Wave.

As before stated, tube ZER of station R is also ionized simultaneously with tube 26W of station W. In this case, however, the cathode element 22R is connected to ground, and the anode element 26R to the conductor HR through the winding of the ringer 25R. Consequently the anode element 26B is now at negative potential and due to its small surface and the consequently small electronic emission therefrom, the current flow from cathode. element 22R to anode element 26R, is so small that the ringer 253 does not respond on the negative half-cycle. noted that the ringers 25-3 and 25M at the stations J and M, respectively, which stations are connected to the tip side of the line, also do not respond at this time, since the tip conductor is connected to ground through a circuit traced from ground through contacts of relay 2?, operated contacts 3d of relay ii and over conductors of plug l and jack 2 to the tip conductor of line Li When the called subscriber of station W responds to the signal of the ringer 25W by lifting his receiver from the switch-hook, the impedance of the circuit is reduced and an increased current flow results, thereby causing the tripping relay 88 to operate over the tip and ring conductors of line Ll. This removes the holding ground from ringing relay H, which thereupon releases and disconnects ringing leads 43 and id from the line, silencing the ringer 25W.

In case the operator desires to ring the R subscriber the key KR. is depressed, which causes consecutive operation of relays 38, 25 and IE, it and El, and 8. The operation of relay 25 opens at its contacts 24, the negative ringing lead 44 previously described and closes, at its contact 39, the positive ringing lead which can be traced from battery PB+, through interrupter I and the right-hand Winding of tripping relay l8, contacts 39 of relay 25, contacts 26 of relay 2?, ringing lead 44, and contacts 28 of relay l! to the It will be established causes tube 25R to ionize.

ring side of the cord and of line Ll. apply a positive potential to conductor ZIR, of station R, and thence through the high resistance 24R to control cathode 23R; The other cathode element 22R. is connected to ground. The potential difference between the two cathodes thus The anode element 25R. is connected through the winding of ringer 25R alsoto conductor HR, and" when tube 20R ionizes there is a flow of current' from anode 26R to cathode 22R and the ringer 25R responds. "Ringer 25W of station W, however, is not actuated, as the anode element 26W is now negative and accordingly passes only a small current, although tube 20W is ionized. When the subscriber of station It answers by removing the receiver from the switch-hook, a low impedance path is connected across the tip and ring conductors, thereby causing the operation of tripping relay l8 through its right-hand Winding. This removes the ground from the winding of relay H, which releases, disconnecting the ringing lead 45, which silences ringer 25R.

Similarly, operation of key KJ operates relays Ml, l5 and El, E6, and i1 and 8. verses the leads 43 and 44 with respect to the contacts of ringing relay I7, thereby causing a negative pulsating potential to be connected to the tip of the cord and of the line, instead of to the ring as in the case of key KW. This negative potential is app-lied to conductor ZIJ, causing tube 28.) with its associated ringer 25J to function in the same manner as did tube 26W and its ringer 25W when negative pulsating current was applied to the ring conductor of line L! Finally, operation of key KM operates relays M, i5, 25 and 21, I6 and H, and 8. Relays 25 and 27, which in this instance are both operated, connect a positive pulsating potential to the tip of the cord over a path which includes the righthand winding of relay l8, contacts 39 of relay 25, contactsdz of relay 21, ringing lead 43 and contacts 34 of relay IT. The aforesaid positive potential is connected to conductor 2 I'M. Tube 20M and its associated ringer 25M now function in the same manner as did tube 25R when a positive pulsating potential was connected to the ring of the line.

The operation of the invention as disclosed by Fig. 3 will now be discussed. This is a subordinate arrangement provided to prevent the improper breakdown of the tubes 30J and 39W of the negative stations J and W when a grounded negative coin battery, the voltage of which excoeds the breakdown voltage of the tube, is con nected at the central ofiice to the tip of the line, for the purpose of operating a grounded coin box device, not shown, which is connected to the tip conductor at each of the stations W, J, M and R.

It will be observed that the connections at stations J and W of the line L2 of Fig. 3 are the same as the corresponding connections of stations J and W of line L! of Fig. 2, excepting that the control cathodes 33J and 33W are not connected to ground but to the side of the line opposite to that over which ringing current for the stations is received. In the case of station J when negative'coin potential is applied to the tip of the line at the central office, in the manner well known in the art and which, therefore, is not shown here, tube 3BJ will not ionize. because the ring conductor of line L2 is not grounded when the aforesaid coin potential is connected to the tip conductor and consequently Relay 21 re-' This is This will the potential drop across the cathode elements 32J and 33J will be insufiicient to cause ionization. The potential drop from anode 36J to cathode 32J, which is the full potential due to the coin currentfis, however, insufficient to ionize the tube, since the breakdown voltage from anode to cathode, as hereinbefore stated, is required to be approximately 150 volts, which is considerably in excess of the coin battery voltage that is ordinarily used.

Although negative station W is connected to' the ring side of the line, it is necessary to make the connections shown in Fig. 3, because with the receiver off the switch-hook, negative coin battery would have access to tube 20W through the switch-hookcontacts. Accordingly, in the case of negative station W, control cathode 33W is connected through its associated high resistance 34W to the tip conductor and cathode 32W to the ring conductor; In this case also the potentials across the cathode elements 33W and 32W, and-across the anode element 35W and cathode element 32W, respectively, are insufficient to ionize the tube. In the case of positive stations M and R, if the negative coin potential should be sufiicient to ionize tubes EtM and 30R, no appreciable current would flow through the ringers 35M and 35R, due to the negative potential applied to the anodes 36M and 3613., and no appreciable current through the cathode circuits due to the high impedances 34M and 34B.

The description of the functioning of line Ll, when ringing currentwas applied by keys KW, KR. KJ and KM is applicable to line L2, except that in the case of stations J and W the breakdown potential across the cathode elements is obtained over the line conductors, the ground at contacts 35 of relay 2'! being used instead of the local ground as in the case of the J and W stations of line LI. 7

A description of the operation of another subordinate feature of the invention as disclosed by Fig. 4 will now be given. In the case of machine ringing it is the usual practice to provide means for operating the tripping relay during the socalled silent period, when ringing current is temporarily disconnected from the line, thereby making the line immediately available when the receiver at the called station is lifted. This is accomplished by the connection of a tripping battery in series with the ringing leads, as soon as the ringing current supply is disconnected. Referring to Fig. 1, the tripping batteries are TB- and TB+. When negative stations J or W are called,,battery TB is connected, in well-known manner, by the interrupter I, in series with the left-hand winding of tripping relay l8, when negative ringing current is disconnected therefrom. In like manner, when positive stations M and R are called, interrupter I connects tripping battery TB+. In some installations the voltage of the batteries TB and TB+ may be sufiiciently high to break down the tubes at the sta tions on the side of the line o er which ringing current is being transmitted if an arrangement similar to that shown by Figs. 2 and 3 is used. To avoid this improper breakdown which might result in the false operation of tripping relay l8, the arrangement of Fig. 4 may be used wherein high impedances 4U, 41W, 41M and 41B, are connected across the cathode elements of tubes IUJ,

49W, 40M and 5 3R.,respectively. When the tripping battery TB is connected to the tip conductor of line L3 of Fig. 4, current flows through high impedances J and 48J in series'over thering conductor back to the ground Connected through the operated contacts 45 of relay 21. The potential across the cathodes'of tube AUJ is therefore equivalent to the voltage drop in impedance llJ. It has been found in practice that impedances 4M and iSJ may be approximately 100,000 ohms and 80,000 ohms, respectively. t is to be understood, however, that these values are not fixed, but should be deter mined by the requirements of each installation. They should be selected so that the drop across the cathode elements is low enough to prevent breakdown of the tube on tripping battery volt age, but still high enough to assure breakdown on ringing current. Accordingly, since the values of impedances ilJ and 58;! are selected to obtain this result, tube lEiJ does not break down. When once the tube has broken down the shunting efiect of the high impedance NJ has little efiect on the operation of the tube and accordingly the performance on ringing current, of tube l'iJ of line L3 is essentially the same as that of tubes 2% and SEIJ of lines L! and L2, which have no cathode shunt.

The functioning of the other tubes of line L3, that is, MSW, 30M and 40B, is the same as that of tube iiEJ.

For the W station of line L3 battery TB is connected over the ring conductor and the circuit is completed through impedances MW and 43W over the tip conductor to ground on the normally closed contacts 35 of relay 27. In the case of the M station tripping battery TB+ is connected over the tip conductor through impedances 48M and MM to local ground at station M, and for station R battery TB+ is connected over the ring conductor through impedances QBR and MB to local ground at station R.

The invention has been described hereinbefore on the basis of using pulsating current derived from the series connection of a positive and negative battery, respectively, in series With a grounded source of alternating ringing current, providing thereby, what is also known in the art as superimposed ringing current. It should also be pointed out that this invention is likewise adapted for use with pulsating current obtained directly from a ringing current source, without the use of a series connected source of direct current. This type of pulsating current is unidirectional with alternate half waves entirely suppressed. Accordingly, with this type of ringing current the tubes connected to the side of the line over which ringing current is being transmitted will be ionized only every alternate half cycle.

It is to be understood that although the invention has been described around certain desirable embodiments thereof, it may also be embodied in various other forms without departing from the spirit of the invention, as defined by the appended claims.

What is claimed is:

1. In a selective signaling system in combination, a line, a plurality of stations on said line, a source of pulsating current, means for connecting said source in either polar direction between either side of the line and ground and for grounding the opposite side of the line, a ringing device responsive to said pulsating current and agesfilled discharge device comprising main and control anodes and a cathode at each station, said discharge devices beingsoconstructed and arranged that current will flow freely between said main anode and cathode when the respective device is ionized and when said mainanode is positive and much less freely when said main anode is negative, and further so constructed 'and'arranged that a higher voltage is required to cause a discharge between said main anode and cathode than between said contro-l'anode and cathode,

and a connection at each station between one side,

of the line and ground including the respective discharge and ringing device, the connection at one station extending from the line to the oathode of the respective discharge device and from the main anode through the respective ringing device to ground with an auxiliary high impedance connection to ground from the control anode, and the connection to ground from the same side of the line at another station extending from the line through the respective ringing device to the main anode of the respective discharge device and from the cathode to ground with a high im-' pedance connection between the same side of the line and the control anode.

2. In a four-party selective signaling system in combination, a plurality of subscribers stations, an electroresponsive ringing device and a gasfilled discharge device comprising main and control anodes and a cathode at each station, a source of pulsating ringing current, means for selectively connecting either pole of said source to either side of the line and ground and the other pole of said source to the opposite side of the line, said ringing devices being adapted to operate in response to said pulsating current and said discharge devices being so constructed and arranged that the main anode-cathode discharge path ofiers a substantially greater impedance to the passage of current in one direction than in the other direction, and a connection at two of said stations from the tip side of the line to round and at the other two stations from the ring side to ground, each connection serially including the respective ringing device and two elements of the respective discharge device, one connection from each line conductor extending serially through the ringing device to the main anode and from the cathode to ground with a high impedance in bridge connection from the respective side of the line to the respective control anode and the other connections from each side of the line extending from the line conductor to the cathode and from the main anode serially through the ringing device to ground with a high impedance connected between the respective control anode and the opposite side of the line.

3. In a selective signaling system, a line, a source of pulsating signal current, selective means for connecting either pole of said source to either side of the line and ground and the other pole of said source to the opposite side of the line, a plurality of electroresponsive ringing devices, each device being connected between a conductor of the line and ground, a three element gas-filled discharge device, comprising an anode, a cathode and a control electrode, associated with each ringing device and so connected that thanodecathode discharge path is in series with the respective ringingdevice, said discharge devices being so constructed and arranged that said anodecathode path has a very substantialunilateral current conducting characteristic and a high impedance bridge connection between each anode and the respective control electrode, and a second high impedance connected between each control electrode and its respective cathode.

4. Ina four-party selective signaling system in combination, a plurality of subscribers stations,

an electroresponsive ringing device and a gasfilled discharge device comprising main and control anodes and a cathode at each station, a source of pulsating ringing current, means for selectively connecting either pole of said source to either side of the line and ground and the other pole of said source to the opposite side of the line, said ringing devices being adapted to operate in response to said pulsating current and said discharge devices being so constructed and arranged that the main anode-cathode discharge path offers ;a substantially greaterimpedance to the passageof current in one direction than in the other direction, and a connection at two of said stations from the tip side of the line to ground and at the other two stations from the ring side to ground, each connection serially including the respective ringing device and two elements of the respective discharge device, one connection from each line conductor extending serially through the ringing device to the main anode and from the cathode to ground with a high impedance in bridge connection from the respective side of the line to the respective control anode and the other connections from each side of the line extending from the line conductor to the cathode and from the main anode serially through the ringing device to ground with a high impedance connected between the respective control anode and the opposite side of the line, and a second high in pedance connected between each control electrode and its respective cathode.

LELAND J. STACY. 

